Conventionally, various structures of a bond magnet rotor of motors have been proposed, which are generally classified into two systems. A first system comprises a so-called surface permanent magnet (referred below to as SPM) rotor, in which permanent magnets are arranged on magnetic pole surfaces as shown in FIGS. 2(a) to 2(c) and 2(f). In contrast, a second system comprises an interior permanent magnet (referred below to as IPM) rotor, in which permanent magnets are arranged in a rotor as shown in FIGS. 2(d) and 2(e). The former SPM rotor is of a type, in which permanent magnets arranged on a rotor surface are opposed to a stator with an air gap therebetween, and which has characteristics in being easy in design and manufacture as compared with the latter IPM rotor. Also, the latter IPM rotor has characteristics in being excellent in structural reliability and easiness, with which reluctance torque is obtained. In addition, an epicyclic type magnet rotor shown in FIG. 2(f) adopts a SPM structure in many cases since there is less risk that magnets centrifugally depart from those base.
For permanent magnet rotors shown in FIG. 2, it is conventionally general to use an adhesive as a way to fix permanent magnets to a surface of, or in a soft magnetic yoke, which is formed from an insulating laminate of silicon steel sheet, casting, forging, or the like.
When a magnet rotor is assembled into a motor to be rotated, there are generated a centrifugal force upon rotation, and magnetic attraction and repulsion between the magnet rotor and a stator. There are also generated vibrations and so on upon rotation. Here, when bonding strength is insufficient between respective magnets and respective soft magnetic yokes, which form a rotor, and between the magnets and the soft magnetic yokes, exfoliation and breakage of the magnets are caused. Since a centrifugal force is increased in substantially proportion to a second power of a rotating speed, such problem becomes serious with high speed rotation. The problem is conspicuous in the case where segment magnets are used as shown in FIG. 2, in particular, in a hypocyclic type SPM rotor, in which magnets are arranged on an outside diametric portion of the rotor as shown in FIGS. 2(a) to 2(c). Further, even in case of using a ring magnet being a single magnet capable of forming a plurality of magnetic poles, a clearance on an adhesive layer is enlarged and a further soft adhesive is used in many cases with a view to avoiding breakage of the magnet due to a difference in coefficient of linear expansion between the magnet and a soft magnetic yoke when a rotor is varied in temperature. In addition, the clearance on the adhesive layer is responsible for an increased dispersion in bonding strength and deviation of positions of bonding. Also, a soft adhesive is generally poor in thermal stability and bonding force. In this manner, there are many technical problems in a bonding work of a magnet rotor irrespective of a shape of a magnet.
From a concern about bonding strength set forth above, as a measure to strength of a hypocyclic type SPM rotor, a protective ring 3 for structural reinforcement, which is made of nonmagnetic stainless steel, fiber reinforced plastic and so on, is in many cases fitted on outer peripheral surfaces of magnets 101 to make up for strength. In such case, however, an effective air gap widens to make magnetic flux from the magnets hard to reach a stator, so that motor output is decreased. Further, a protective ring made of metal such as stainless steel, etc. generates an eddy current loss to lead to a decrease in motor efficiency. In addition, it is apparent that no sufficient bonding strength is obtained between magnets and a soft magnetic yoke since use of a frame and a protective ring for structural reinforcement is premised in JP-A-2001-95185 (Patent Document 1) and JP-A-2003-32931 (Patent Document 2) listed as comparative examples, in which magnets and a soft magnetic yoke are formed integrally. Also, since a ring magnet is wedge-shaped to bite into a yoke owing to macroscopic, outward appearance and shape of the magnet to prevent coming-off from the soft magnetic yoke in JP-A-5-326232 (Patent Document 3) and a magnet is limited to a ring shape in JP-A-7-169633 (Patent Document 4), it is apparent from descriptions regarding a manufacturing method that no sufficient bonding strength is obtained between a magnet and a soft magnetic yoke and the soft magnetic yoke is held only by inner pressure in a ring magnet. In JP-A-2001-052921 (Patent Document 5), temporarily forming under compression and main forming are performed to form a ring-shaped magnet. However, the ring-shaped magnet and a soft magnetic yoke are bonded together by adhesion to be insufficient in bonding strength and reliability.
By the way, permanent magnets include two types of isotropic and anisotropic ones. Isotropic magnets are 20% lower in magnetic properties than anisotropic magnets but have a character that manufacture is easy since there is no need of giving any magnetic field in a process of compression-forming magnetic powder. On the other hand, by charging raw material powder, which possesses axes of easy magnetization and is put in nonmagnetized state, into a metallic die, giving thereto a strong magnetic field in a suitable way to arrange the axes of easy magnetization in a specified direction, and compression-forming in an intact state and sintering, or hardening with a thermosetting resin, an anisotropic magnet is little changed in properties and functions as a permanent magnet. Here, with an anisotropic bond magnet of ferrite or rare-earth, after a raw material is pulverized, magnetic powder 6 is formed under compression in a metallic die, to which a magnetic field is applied as shown in FIG. 4 (arrows A in the drawing indicate a pressing direction). Thereby, from the magnetized magnetic powder 6, powder magnets having magnetic poles N, S along an axis of easy magnetization are produced so as to be generally in conformity with an external magnetic field like a compass needle. When forming under compression is performed in this state, a green compact with aligned axes of easy magnetization is resulted. In addition, an anisotropic magnet is subjected to demagnetization treatment by application of a backing field, or an alternating attenuation magnetic field in the last process of forming in a magnetic field. The green compact is beforehand mixed with a thermosetting resin and subjected to heat curing to be made a bond magnet. A magnet with axes of easy magnetization arranged in this manner is said to be an anisotropic magnet. For an anisotropic magnet, excellent, magnetic properties are obtained only in a direction, in which axes of easy magnetization are aligned.    Patent Document 1: JP-A-2001-95185    Patent Document 2: JP-A-2003-32931    Patent Document 3: JP-A-5-326232    Patent Document 4: JP-A-7-169633    Patent Document 5: JP-A-2001-052921